Use template for lower-constraints

Small cleanups:

* Use openstack-lower-constraints-jobs template, remove individual
  jobs.
* Sort list of templates

Change-Id: I4c107cd94d4e712d2a14eb2b2b68dbe12701322d
Needed-By: https://review.openstack.org/623229

zuul.d/layout.yaml

@@ -1,20 +1,15 @@
 - project:
     templates:
-      - tripleo-undercloud-jobs
-      - tripleo-multinode-container-minimal
-      - tripleo-buildimage-jobs
+      - check-requirements
+      - openstack-lower-constraints-jobs
+      - openstackclient-plugin-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
       - openstack-python37-jobs
-      - check-requirements
-      - openstackclient-plugin-jobs
-      - release-notes-jobs-python3
       - publish-openstack-docs-pti
+      - release-notes-jobs-python3
+      - tripleo-buildimage-jobs
       - tripleo-build-containers-jobs
-    check:
-      jobs:
-        - openstack-tox-lower-constraints
-    gate:
-      jobs:
-        - openstack-tox-lower-constraints
+      - tripleo-multinode-container-minimal
+      - tripleo-undercloud-jobs